1. Field of the Invention
Exemplary embodiments of the present invention relate to a method and an apparatus for precise positioning using Global Positioning System (GPS) pseudolites in a space where no GPS signal can reach (e.g. indoor environment).
2. Description of Related Art
In general, a GPS system refers to a system for determining positions using satellites placed in a medium Earth orbit at a predetermined altitude from the Earth. Specifically, a user of the GPS system receives signals from at least four different satellites in the medium Earth orbit, and calculates his/her position using the received signals. Besides the current position, altitude information can also be acquired, if necessary, from the signals.
However, the GPS system has a limitation in that signals from the medium Earth orbit satellites cannot reach indoor environments or urban/canyon areas with densely located tall buildings and mountains, making the GPS positioning service unavailable.
Therefore, as an alternative to the GPS system in indoor environments or urban/canyon areas with densely located tall buildings and mountains, a method of using pseudolites has been proposed. The pseudolites will now be described briefly.
Pseudolites refer to devices for transmitting signals to indoor environments or areas where densely located tall buildings block signals from medium Earth orbit satellites, as if the signals are transmitted from the satellites.
The pseudolite system includes a number of pseudolites and a reference station for monitoring signals of the pseudolites and generating calibration information (e.g. time calibration information). The pseudolite system requires a wireless link to transmit calibration information from the reference station to users.
An advanced type of pseudolite system includes a master pseudolite and a number of slave pseudolites. This type of pseudolite system equips the master pseudolite with a precise time source so as not to use a wireless link for providing users with calibration information (e.g. time calibration information). A reference station, which is configured to monitor pseudolite signals, monitors the time of the slave pseudolites. A system has also been proposed in which, based on information regarding the time of a number of slave pseudolites monitored by a reference station, each slave pseudolite is synchronized by a PLL control circuit installed in the pseudolite. Such a pseudolite system including master and slave pseudolites is too expensive to be used for indoor positioning because a system of a number of pseudolites must be installed on each floor.